In contrast to conventional IC assemblies that integrate a number of IC chips or die having millimeter lateral chip dimensions onto a substrate, micro device assemblies integrate electronic and/or photonic devices having lateral chip dimensions in the micrometer regime (e.g., 1-10 μm). Die isolation, handling, bonding, and interconnection are all more of a challenge for micro device assemblies than conventional IC assemblies. For example, traditional pick-and-place methods can handle devices down to ˜200 μm in the lateral (XY) dimensions and 100 μm in thickness (Z). Due to this large chip thickness, it is difficult to form interconnects that allow miniaturization of the package.
Micro device assembly techniques under development include so-called transfer printing methods that have been shown capable of integrating into an assembly micro dice having lateral dimensions in the 10's of micrometers and ˜1 μm in Z thickness. One exemplary transfer printing technique relies on a sophisticated MEMS print head providing thousands of electrostatic clamping points. Another exemplary transfer printing technique relies on low temperature bonding with polydimethylsiloxane (PDMS), which is not compatible with high temperature chip bonding techniques, like solder bonding.
Electronic displays are one area that may benefit from advanced micro device assembly techniques. Electronic display technology has advanced rapidly in recent years as an important user interface to electronic devices. To date, liquid crystal display (LCD) technology has been the dominant display technology for both large format (e.g., television) and mobile devices. Current LCD based displays however only pass through ˜5% of light from a backlight source (e.g., LED or CFL, etc.) leading to poor power efficiency, insufficient daylight display illumination, and poor viewing angles.
Considerable research and development has been expended on organic light emitting diode (OLED) display technology. OLED displays improve display power efficiency, though not dramatically, relative to LCD. OLED technology also currently suffers from color fading, leading to decreased display endurance/lifetime. Another next-gen display technology under investigation is crystalline LED, also referred to as inorganic LED (iLED). A crystalline LED display relies on an array of crystalline semiconductor LED chips. A crystalline LED display, for example, may utilize one LED chip for one picture element, or pixel. The power efficiency of crystalline LED is one order of magnitude more efficient than that of OLED, however a high volume manufacturing process has not been demonstrated for display applications. One of the technical challenges of crystalline LED is that a vast number of very small crystalline LEDs need to be transferred from a monolithic growth/fabrication medium into a spatially larger array electrically interconnected in a manner that enables controlled light emission. For current display resolutions (e.g., HD), one may expect hundreds of thousands of crystalline LED elements within a 1″ square of display area with each crystalline LED element on the micron scale (e.g., 5 μm, or less on a side).
Micro-bonding techniques capable of assembling crystalline LED displays are therefore advantageous.